1. Field of the Invention
The present invention generally relates to the field of medicinal chemistry. More specifically, the present invention relates to the use of synthetic carotenoid analogs, derivatives and compositions made using same for the treatment and prevention of platelet aggregation and thrombus formation in a subject.
2. Description of the Relevant Art
Platelet accumulation at sites of vascular injury is a dynamic process that mediates formation of both the primary hemostatic plug and pathologic thrombus formation. The mechanisms by which platelet surface proteins direct platelet recruitment to thrombi under flow conditions have been studied in detail. In addition to directing initial platelet adhesion, cell-surface receptor interactions activate intracellular signaling. Intracellular signaling stimulates the release of thrombogenic substances from platelet granules. Signaling also mediates activation of the platelet integrin αIIbβ3 (gpIIb-IIIa) that facilitates firm adhesion of platelets to thrombi.
Arterial thrombosis mediates tissue infarction in coronary artery disease, cerebrovascular disease, and peripheral vascular disease, and, thus, is the single most common cause of morbidity and mortality in the United States. Platelets are key mediators of arterial thrombosis. Thus, the identification of compounds that inhibit platelet function is of great importance to medicine.
Platelets form the body's primary means of hemostasis and, as such, have developed an elaborate mechanism of surveying the vasculature for defects in endothelial integrity. This mechanism involves the ability to respond to subendothelial matrices, shear forces, neighboring platelets, the adrenal axis, as well as soluble proteinacious, nucleotide, and lipid signals. Despite this plethora of physiologic activators, the platelet has only a small repertoire of major functional outputs. Upon activation, platelets change shape, aggregate, and secrete their granular contents. The process of platelet activation involves the expression of activities not shared by functionally merit resting platelets, including, for example, ATP release, serotonin release, lysosomal release, alpha granule release, dense granule release, and cell surface expression of markers of activated platelets (including, but not limited to CD9, gpIb, gpIIb, gpIIIa, CDIa-IIa, P-selectin, PECAM-1, activated gpIIb/IIIa, and vitronectin receptor). In addition, platelet activation results in the aggregation of platelets with each other and with non-platelet surrounding cells. The granular contents of platelets supply additional adhesion molecules, growth factors, coagulation enzymes and other specialized molecules instrumental in the process of thrombus formation and the initiation of the healing process.
In addition to coronary artery disease/myocardial infarction, cerebrovascular disease and peripheral vascular disease, diseases and disorders associated with inappropriate platelet activity and arterial thrombosis also include, for example, stable and unstable angina, transient ischemic attacks, placental insufficiency, unwanted thromboses subsequent to surgical procedures (e.g., aortocoronary bypass surgery, angioplasty and stent placement, and heart valve replacement), or thromboses subsequent to atrial fibrillation. Inhibitors of platelet activity can provide therapeutic and preventive benefits for each of these diseases or disorders. It is also possible that inappropriate platelet activation plays a role in venous thrombosis, such that platelet inhibitors can be useful for the treatment or prevention of disorders associated with such thromboses.
A connection is emerging between platelet activation and inflammation, particularly allergic inflammation (e.g., in asthma) and inflammation at the sites of atherosclerotic damage. Therefore, compounds that inhibit platelet activation can also be useful in the treatment or prevention of disorders involving inflammation.
There are a number of agents presently available that target platelet function. For example, aspirin is a relatively weak platelet inhibitor. However, aspirin can cause life-threatening allergic reactions in sensitive individuals.
Another platelet inhibiting agent is ticlopidine (TICLID™, Roche Pharmaceuticals). Because it requires the production of active metabolites to be effective, the effect of ticlopidine is delayed 24-48 hours. The drug can also cause thrombotic thrombocytopenic purpura, a life-threatening condition, as well as nausea, abdominal pain, dyspepsia, diarrhea and skin rash.
Clodiprogel (PLAVIX™, Bristol-Meyers Squibb/Sanofi Pharmaceuticals) is another platelet inhibitor that requires the generation of active metabolites for its therapeutic efficacy. Therefore, clodiprogel also has a delay of 24-48 hours for its effect. Clodiprogel can also cause thrombotic thrombocytopenia purpura as well as agranulocytopenia, both life-threatening conditions. The drug has also been associated with rash, edema, hypertension, hypercholesterolemia, nausea, abdominal pain, dyspepsia, diarrhea, urinary tract infections, liver enzyme elevations and arthralgia.
The platelet inhibitory agents Abiximab and c7E3 Fab (REOPRO ABCIXIMAB™, manufacturer—Centocor B.V., distributor—Eli Lilly and Co.) are only available in a parenteral form. The drugs can cause severe thrombocytopenia. Both have a very long half-life and, therefore, complicate surgery that is sometimes required in the setting of life-threatening arterial occlusion (e.g., emergent cardiac surgery in the setting of a myocardial infarction).
Tirofiban (AGGRASTAT™, Merck and Co., Inc.) is another platelet inhibitory agent that is only available in a parenteral form. Tirofiban can cause thrombocytopenia, coronary artery dissection, bradycardia and edema, as well as dizziness and vasovagal reactions.
Eptifibatide (INTEGRILIN™, COR Therapeutics, Inc., Key Pharmaceuticals Inc.) is another platelet inhibitory agent that is only available for parenteral administration. It can cause hypotension.
There is only limited clinical experience with the oral anti-GPIIbIIIa agents lamifiban, sibrafiban, orofiban and xemilofiban. Similarly, clinical experience is limited with the phosphodiesterase inhibitors cilostazol, trapidil and trifusal. There is more clinical experience with the phosphodiesterase inhibitor dipyridamole, but its activity is so weak that it is not frequently used.
There is a need in the art for additional platelet inhibitory agents for the treatment and prevention of diseases or disorders characterized by platelet activation and thrombosis. There is a need in the area of cardiovascular and cerebrovascular therapeutics for an agent that can be used in the prevention and treatment of thrombi, with minimal side effects, such as unwanted prolongation of bleeding, while preventing or treating target thrombi.
Antioxidant Properties of Carotenoids
Carotenoids are a group of natural pigments produced principally by plants, yeast, and microalgae. The family of related compounds now numbers greater than 750 described members, exclusive of Z and E isomers. Humans and other animals cannot synthesize carotenoids de novo and must obtain them from their diet. All carotenoids share common chemical features, such as a polyisoprenoid structure, a long polyene chain forming the chromophore, and near symmetry around the central double bond. Tail-to-tail linkage of two C20 geranyl-geranyl diphosphate molecules produces the parent C40 carbon skeleton. Carotenoids without oxygenated functional groups are called “carotenes”, reflecting their hydrocarbon nature; oxygenated carotenes are known as “xanthophylls.” “Parent” carotenoids may generally refer to those natural compounds utilized as starting scaffold for structural carotenoid analog synthesis. Carotenoid derivatives may be derived from a naturally occurring carotenoid. Naturally occurring carotenoids may include lycopene, lycophyll, lycoxanthin, astaxanthin, beta-carotene, lutein, zeaxanthin, and/or canthaxanthin to name a few.
Cyclization at one or both ends of the molecule yields 7 identified end groups (illustrative structures shown in FIG. 1). Examples of uses of carotenoid derivatives and analogs are illustrated in U.S. patent application Ser. No. 10/793,671 filed on Mar. 4, 2004, entitled “CAROTENOID ETHER ANALOGS OR DERIVATIVES FOR THE INHIBITION AND AMELIORATION OF DISEASE” by Lockwood et al. published on Jan. 13, 2005, as Publication No. US-2005-0009758 and PCT International Application Number PCT/US2003/023706 filed on Jul. 29, 2003, entitled “STRUCTURAL CAROTENOID ANALOGS FOR THE INHIBITION AND AMELIORATION OF DISEASE” by Lockwood et al. (International Publication Number WO 2004/011423 A2, published on Feb. 5, 2004) both of which are incorporated by reference as though fully set forth herein.
Documented carotenoid functions in nature include light harvesting, photoprotection, and protective and sex-related coloration in microscopic organisms, mammals, and birds, respectively. A relatively recent observation has been the protective role of carotenoids against age-related diseases in humans as part of a complex antioxidant network within cells. This role is dictated by the close relationship between the physicochemical properties of individual carotenoids and their in vivo functions in organisms. The long system of alternating double and single bonds in the central part of the molecule (delocalizing the π-orbital electrons over the entire length of the polyene chain) confers the distinctive molecular shape, chemical reactivity, and light-absorbing properties of carotenoids. Additionally, isomerism around C═C double bonds yields distinctly different molecular structures that may be isolated as separate compounds (known as Z (“cis”) and E (“trans”), or geometric, isomers). Of the more than 750 described carotenoids, an even greater number of the theoretically possible mono-Z and poly-Z isomers are sometimes encountered in nature. The presence of a Z double bond creates greater steric hindrance between nearby hydrogen atoms and/or methyl groups, so that Z isomers are generally less stable thermodynamically, and more chemically reactive, than the corresponding all-E form. The all-E configuration is an extended, linear, and rigid molecule. Z-isomers are, by contrast, not simple, linear molecules (the so-called “bent-chain” isomers). The presence of any Z in the polyene chain creates a bent-chain molecule. The tendency of Z-isomers to crystallize or aggregate is much less than all-E, and Z isomers are more readily solubilized, absorbed, and transported in vivo than their all-E counterparts. This has important implications for enteral (e.g., oral) and parenteral (e.g., intravenous, intra-arterial, intramuscular, and subcutaneous) dosing in mammals.
Carotenoids with chiral centers may exist either as the R (rectus) or S (sinister) configurations. As an example, astaxanthin (with 2 chiral centers at the 3 and 3′ carbons) may exist as 4 possible stereoisomers: 3S, 3′S; 3R, 3′S and 3S, 3′R (identical meso forms); or 3R, 3′R. The relative proportions of each of the stereoisomers may vary by natural source. For example, Haematococcus pluvialis microalgal meal is 99% 3S, 3′S astaxanthin, and is likely the predominant human evolutionary source of astaxanthin. Krill (3R,3′R) and yeast sources yield different stereoisomer compositions than the microalgal source. Synthetic astaxanthin, produced by large manufacturers such as Hoffmann-LaRoche AG, Buckton Scott (USA), or BASF AG, are provided as defined geometric isomer mixtures of a 1:2:1 stereoisomer mixture [3S, 3′S; 3R, 3′S, 3′R,3S (meso); 3R, 3′R] of non-esterified, free astaxanthin. Natural source astaxanthin from salmonid fish is predominantly a single stereoisomer (3S,3′S), but does contain a mixture of geometric isomers. Astaxanthin from the natural source Haematococcus pluvialis may contain nearly 50% Z isomers. As stated above, the Z conformational change may lead to a higher steric interference between the two parts of the carotenoid molecule, rendering it less stable, more reactive, and more susceptible to reactivity at low oxygen tensions. In such a situation, in relation to the all-E form, the Z forms: (1) may be degraded first; (2) may better suppress the attack of cells by reactive oxygen species such as superoxide anion; and (3) may preferentially slow the formation of radicals. Overall, the Z forms may initially be thermodynamically favored to protect the lipophilic portions of the cell and the cell membrane from destruction. It is important to note, however, that the all-E form of astaxanthin, unlike β-carotene, retains significant oral bioavailability as well as antioxidant capacity in the form of its dihydroxy- and diketo-substitutions on the β-ionone rings, and has been demonstrated to have increased efficacy over β-carotene in most studies. The all-E form of astaxanthin has also been postulated to have the most membrane-stabilizing effect on cells in vivo. Therefore, it is likely that the all-E form of astaxanthin in natural and synthetic mixtures of stereoisomers is also extremely important in antioxidant mechanisms, and may be the form most suitable for particular pharmaceutical preparations.
The antioxidant mechanism(s) of carotenoids, and in particular astaxanthin, includes singlet oxygen quenching, direct radical scavenging, and lipid peroxidation chain-breaking. The polyene chain of the carotenoid absorbs the excited energy of singlet oxygen, effectively stabilizing the energy transfer by delocalization along the chain, and dissipates the energy to the local environment as heat. Transfer of energy from triplet-state chlorophyll (in plants) or other porphyrins and proto-porphyrins (in mammals) to carotenoids occurs much more readily than the alternative energy transfer to oxygen to form the highly reactive and destructive singlet oxygen (1O2). Carotenoids may also accept the excitation energy from singlet oxygen if any should be formed in situ, and again dissipate the energy as heat to the local environment. This singlet oxygen quenching ability has significant implications in cardiac ischemia, macular degeneration, porphyria, and other disease states in which production of singlet oxygen has damaging effects. In the physical quenching mechanism, the carotenoid molecule may be regenerated (most frequently), or be lost. Carotenoids are also excellent chain-breaking antioxidants, a mechanism important in inhibiting the peroxidation of lipids. Astaxanthin can donate a hydrogen (H) to the unstable polyunsaturated fatty acid (PUFA) radical, stopping the chain reaction. Peroxyl radicals may also, by addition to the polyene chain of carotenoids, be the proximate cause for lipid peroxide chain termination. The appropriate dose of astaxanthin has been shown to completely suppress the peroxyl radical chain reaction in liposome systems. Astaxanthin shares with vitamin E this dual antioxidant defense system of singlet oxygen quenching and direct radical scavenging, and in most instances (and particularly at low oxygen tension in vivo) is superior to vitamin E as a radical scavenger and physical quencher of singlet oxygen.
Carotenoids, and in particular astaxanthin, are potent direct radical scavengers and singlet oxygen quenchers and possess all the desirable qualities of such therapeutic agents for inhibition or amelioration of ischemia-reperfusion (I/R) injury. Synthesis of novel carotenoid derivatives with “soft-drug” properties (i.e. activity in the derivatized form), with physiologically relevant, cleavable linkages to pro-moieties, can generate significant levels of free carotenoids in both plasma and solid organs. This is critically important, for in mammals, diesters of carotenoids generate the non-esterified or “free” parent carotenoid, and may be viewed as elegant synthetic and novel delivery vehicles with improved properties for delivery of free carotenoid to the systemic circulation and ultimately to target tissue. In the case of non-esterified, free astaxanthin, this is a particularly useful embodiment (characteristics specific to non-esterified, free astaxanthin below):                Lipid soluble in natural form; may be modified to become more water soluble        Molecular weight of 597 Daltons [size<600 daltons (Da) readily crosses the blood brain barrier, or BBB]        Long polyene chain characteristic of carotenoids effective in singlet oxygen quenching and lipid peroxidation chain breaking        No pro-vitamin A activity in mammals (eliminating concerns of hypervitaminosis A and retinoid toxicity in humans).        
The administration of antioxidants that are potent singlet oxygen quenchers and direct radical scavengers, particularly of superoxide anion, should limit hepatic fibrosis and the progression to cirrhosis by affecting the activation of hepatic stellate cells early in the fibrogenetic pathway. Reduction in the level of ROS by the administration of a potent antioxidant can therefore be crucial in the prevention of the activation of both HSC and Kupffer cells. This protective antioxidant effect appears to be spread across the range of potential therapeutic antioxidants, including water-soluble (e.g., vitamin C, glutathione, resveratrol) and lipophilic (e.g., vitamin E, β-carotene, astaxanthin) agents. Therefore, a co-antioxidant derivative strategy in which water-soluble and lipophilic agents are combined synthetically is a particularly useful embodiment.
Vitamin E is generally considered the reference antioxidant. When compared with vitamin E, carotenoids are more efficient in quenching singlet oxygen in homogeneous organic solvents and in liposome systems. They are better chain-breaking antioxidants as well in liposomal systems. They have demonstrated increased efficacy and potency in vivo. They are particularly effective at low oxygen tension, and in low concentration, making them extremely effective agents in disease conditions in which ischemia is an important part of the tissue injury and pathology. These carotenoids also have a natural tropism for the liver after oral administration. Therefore, therapeutic administration of carotenoids should provide a greater benefit in limiting fibrosis than vitamin E.
Problems related to the use of naturally occurring carotenoids and some structural carotenoid analogs include: (1) the complex isomeric mixtures, including non-carotenoid contaminants, provided in natural and synthetic sources leading to costly increases in safety and efficacy tests required by such agencies as the FDA; (2) limited bioavailability upon administration to a subject; and (3) the differential induction of cytochrome P450 enzymes (this family of enzymes exhibits species-specific differences which must be taken into account when extrapolating animal work to human studies).